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Abstract

In recent years, a lot of work on the development of oil shale has been done with regard to the Chinese oil shales. However, the research is mainly with respect to the processing aspects of oil shale. The pyrolysis mechanism of oil shale has been limited, to some extent, by instrumentation and analytical techniques. In this paper, pyrolysis kinetics characteristics of oil shale samples obtained from Mudanjiang were investigated by Rock-Eval analysis. The weight loss data, pyrolysis mechanism, and properties of oil shale were analyzed at different heating rates (10, 15, 20, 25, 30℃/min) in the temperature range from 200 to 600℃. In this paper, a distribution function of apparent activation energy and a relationship equation of apparent energy with frequency factor have been obtained. A special kinetic model was developed, fully suitable for the pyrolysis characteristics of oil shale. This model was also verified by experiment. Throughout the analysis of pyrolysis kinetics, it was observed that the pyrolysis reaction mechanism at different temperatures could be explained. The results may provide the theoretical basis for further effective and economical exploitation of oil shale.

Keywords

Mathematical model oil shale pyrolysis mechanism Songliao Basin industrial property

Oil shale is an organic-rich fine-grained sedimentary rock containing kerogen from which liquid hydrocarbons called shale oil can be produced. Deposits of oil shale occur around the world, including major deposits in the United States, China, and others. In China, oil shale deposits are widespread in many regions, and the proved reserves amount to about 720 billion tons presenting a potential energy source (Bai et al., 2015; Hou, 1988; Liu et al., 2009; Ning et al, 2011; Wang, 2011; Zou et al., 2015;). According to the evaluation result, the measured reserves account for two-thirds in PetroChina Company Limited. The pilot bases of 30 thousand tons shale oil have been constructed in Mudanjiang on 2012 (Gerasimov, 2015; Kang et al., 2008; Liu, 2005, 2009; Lei, 2008).

In this work, a study on Mudanjiang oil shale in the Songliao Basin was carried out to investigate the effect of heating rate on the pyrolysis of oil shale by employing Rock-Eval technique. The pyrolysis temperature ranges from room temperature to 900℃. The industrial property of Mudanjiang oil shale was determined and the mechanism of oil shale pyrolysis was tentatively investigated. Through analysis and comparison of the experimental results, the effect of heating rate and kinetics on the pyrolysis behavior of oil shale was investigated, to have access to information to be taken advantage of in the utilization of oil shale (Abbassi et al., 2015; Aboulkas et al., 2008; Lin et al., 2016; Ren et al., 2015; Sun et al., 2015; Tiwari et al., 2012).

Industrial property of Mudanjiang oil shale

In order to further optimize the process conditions and retort dimensions, there is a clear need to investigate the oil shale industrial property. Industrial property of oil shale includes oil yield, water yield, calorific value, and mechanical strength, which has impacts on the industrial application of oil shale.

The ISO 647 standard describes a method for the pyrolysis of oil shale that quantitatively yields water, gas, tar and char, also known as a Fischer assay. In the ISO 647 method, an oil shale sample is heated according to a heat curve that specifies the temperature over intervals of 10 min for a total time of 80 min, up to 520℃. During this process, the oil shale converts to char and condensable volatilities (tar/oil and water).The water fraction is removed by azeotropic distillation using a Dean and Stark distillation method with the use of toluene as a solvent. The toluene can be removed by rotary evaporation to obtain the tar. The char, tar/oil, water along with the gas content (determined by difference) constitute the mass of the original sample. The Fischer assay method is widely used in the oil shale pyrolysis and liquefaction research.

Thirty-three oil shale sample used in this work were selected from Mudanjiang, China. The proximate analysis, Fischer assay, and property analysis of oil shale were performed according to the universal standards. Calorific value was determined by the oxygen bomb method.

The properties of Mudanjiang oil shale are shown in Table 1. As seen from the table, oil yield of Mudanjiang oil shale is high, which becomes a restricted factor for oil shale utilization. The oil yield is 12.49% compared to Maoming oil shale (7%) and Funsun oil shale (6%), which indicates that Mudanjiang oil shale has a high grade industrial value. As observed from the table, there exists as high as water yield of 27.14% in the Mudanjiang oil shale, compared to Maoming oil shale (17%) and Funsun oil shale (5%). Because of the large specific heat and more moisture content, more energy is needed for the pyrolysis process of the oil shale. This will bring about a lot of inconvenience to the commercial plant. From Table 1 it is seen that the Mudanjiang oil shale has low ash content (35.85%) compared to Funsun oil shale (71.44%). Ash evaluation is an important indicator of oil shale. The lower ash content means the lower remaining residue. This will reduce the operation load of the commercial plant. The calorific value of Mudanjiang oil shale was high (13.86 MJ/kg (received)) when compared to that of Funsun oil shale (5 MJ/kg) and Maoming (6 MJ/kg). The calorific value is an important quality indicator of oil shale’s comprehensive utilization. It is calculated for heat balance, consumption, and thermal efficiency of the process of combustion (Brown et al., 2000; Liu, 2006; Starink et al., 2003; Uguna et al., 2012, 2016).

Table 1.

Industrial property of Mudanjiang oil shale.

Sample	Oil yield (%)	Water content (%)	Ash content (%)	Calorific value (MJ/kg)
Sample 1	5.39	27.05	61.93	7.16
Sample 2	11.45	36.02	42.43	19.99
Sample 3	10.02	30.78	51.56	10.75
Sample 4	10.97	30.28	50.57	15.72
Sample 5	11.49	31.04	38.67	23.40
Sample 6	12.54	36	41.01	20.61
Sample 7	12.24	36.67	40.03	25.60
Sample 8	7.13	41.16	41.72	14.12
Sample 9	7.78	37.68	45.02	15.66
Sample 10	6.52	27.54	59.32	10.56
Sample 11	10.79	11.56	56.22	16.83
Sample 12	11.04	32.27	45.59	20.33
Sample 13	0.81	17.14	80.94	0.40
Sample 14	9.62	33.93	48.49	15.70
Sample 15	9.84	30.02	52.32	12.74
Sample 16	13.71	35.58	44.24	14.89
Sample 17	8.48	25.6	49.54	11.92
Sample 18	9.1	32.8	44.96	16.82
Sample 19	13.45	33.96	42.91	14.40
Sample 20	9.57	40.35	41.35	15.86
Sample 21	1.53	37.33	57.9	3.74
Sample 22	12.9	42.58	36.27	17.30
Sample 23	9.07	38.29	44.26	19.18
Sample 24	3.66	18.71	75.06	3.93
Sample 25	6.88	41.03	41.51	20.65
Sample 26	11.26	38.79	43.96	21.29
Sample 27	15.26	36.69	36.63	20.47
Sample 28	11.09	33.51	46.71	15.56
Sample 29	11.2	37.73	40.38	19.46
Sample 30	17.71	34.43	38.12	21.60
Sample 31	7.93	38.05	39.27	16.21
Sample 32	11.41	36.06	40.73	17.06
Sample 33	9.29	37.94	41.2	21.30

Table 2 shows the mechanical characteristics of Mudanjiang oil shale. The mechanical strength of Mudanjiang oil shale is 1000 kPa, which is lower than that of Fushun (7800 kPa) indicating that Mudanjiang oil shale has a low mechanical strength. Owing to this, the Mudanjiang oil shale easily breaks into small pieces in the process of oil shale retorting. It also increases the resistance of air, causing uneven distribution of gas, which impacts the operation of retort. Hence, the four characteristics of Mudanjiang oil shale namely high oil yield, high water yield, high calorific value, and low mechanical strength, represent “three-highs and one-low”.

Table 2.

Mechanical strength of oil shale.

Sample	Diameter (mm)	Pressure (kN)	Uniaxial compressive strength (kPa)
MD108	58.30	3.6	1348.58
MD105	60.15	1.5	527.87
MD109	63.97	1.5	466.71
Fushun	150*100	117	7800

Pyrolysis mechanism of oil shale

The organic matter of oil shale is composed of aliphatics, aromatics, and some heteroatomic functional group, which causes pyrolysis. There are mainly three kinds of chemical reactions in the pyrolysis process: reaction of aliphatics, aromatics, and heteroatomic functional group (Abbassi et al., 2014; Li, 1986; Liu et al., 1987; Xie et al., 2015; Wang et al., 2015; Zadeh et al., 2016).

Reaction of aliphatics

The pyrolysis reactions of aliphatic hydrocarbons are the carbon–carbon and carbon–hydrogen bond’s rupture of n-alkanes, isoparaffin, cycloalkane, olefin, which are endothermic.

Reaction of aromatics

The reaction of aromatics contains dehydrogenation, condensation, and carbonation reaction of alkylate, which are exothermic.

Reaction of heteroatomic functional group

The reaction of heteroatomic functional group contains decarboxylation, dehydroxylation, deamination, and desulphation. This reaction determines the content of NH₃, CO₂, and element of S, N, and has little influence on the quality of shale oil.

① Decarboxylation $RCOOH \to RH + {CO}_{2}$

② Dehydroxylation ${RCH}_{2} {CH}_{2} OH \to RCH = {CH}_{2} + H_{2} O$

③ Deamination

④ Desulphation

Kinetics of thermal decomposition

Oil shale sample used in this work was obtained from Mudanjiang, China. It was ground and sieved to corresponding sizes for different uses. Pyrolysis experiments were carried out using a TA SDT-Q600 analyzer, with the heating rate of 20℃/min to a final temperature of 900℃. About 20 mg of oil shale was put into the sample crucible. About 80 mL/min of high-purity nitrogen as a carrier gas was made to flow through the reactor during the experiment.

Figure 1 shows the weight loss (TG) curves of oil shale in relation to the heating rate to the final temperature of 900℃. It can be seen that there are three stages in the TG figures of pyrolysis process of oil shale.

Figure 1.

TG curves of oil shale.

The first stage is called the low-temperature weight loss step (stage 1), and it occurs in the range of room temperature to 200℃. It is mainly caused by the precipitation of internal water and the layer water of clay mineral. It accounts for 5% of the total weight loss.

The second stage is called the decomposition of OM (stage 2), and it occurs in the range of 200–600℃. During stage 2, weight loss occurs that has been attributed to the decomposition of hydrocarbons, with the escaping of oil and gas steam. This section accounts for about 70% of the total weight loss.

The last stage is called pyrolysis of carbonate (stage 3), and it occurs in the range of 600–900℃. During stage 3, the weight loss is mainly caused by the decomposition of clay and carbonate minerals. The rate of weight loss is distinctly lower than that of the second stage (Gao et al., 2016; Hackley et al., 2016; Patel et al., 2013; Wang et al., 2012).

The Rock-Eval pyrolysis method has been widely used for oil and gas exploration in sedimentary basins all over the world. This technique uses temperature programmed heating of a small amount of oil shale in an inert atmosphere (helium or nitrogen) in order to determine the quantity of free hydrocarbons present in the sample and of those that can be potentially released after pyrolysis.

The samples were pyrolyzed at the heating rates of 10℃/min, 15℃/min, 20℃/min, 25℃/min, and 30℃/min, respectively. The pyrolysis experiment was performed in Rock-Eval6 instrument. The pyrolysis temperatures ranged from 200℃ to 1000℃ at different heating rates to obtain the hydrocarbon yield. Conversion and conversion rate of hydrocarbons versus temperature were taken under nonisothermal condition.

The pyrolysis process of oil shale is very complicated. The intrinsic kinetic data were obtained in a Rock Eval. The “three dynamic factors” were calculated by Coats–Redferm method, Friedman method, Flynn–Wall–Ozawa method, and Kissinger–Akahira–Sunose method, and the reaction dynamics mathematical model was given.

The data on the relationship between conversion and temperature can be obtained. The graph is plotted for conversion versus temperature for various heating rates and is shown in Figure 2. A conversion value at a particular temperature can be obtained from this figure for all the heating rates. As the temperature increases, the conversion rate of hydrocarbons also increases, because the processes of the pyrolysis of hydrocarbons are continuous and irreversible. The curves are flat in the low-temperature stage and are steeper in the high temperature stage. Figure 3 shows that with the rise in the heating rate, there is a gradual increase in the conversion rate of hydrocarbons, and a lag in the maximum temperature.

Figure 2.

Conversion of hydrocarbons at various temperatures.

Figure 3.

Conversion rate of hydrocarbons at various temperatures.

Figure 4.

Active energy at various conversion rates.

Discussion

Kinetics of the thermal decomposition

The relation between the conversion and weight loss data was obtained by the following equation $x (%) = \frac{W_{0} - W_{T}}{W_{0} - W_{f}}$ (1) where W₀, W_T, and W_f are the initial weight of oil shale, the weight at temperature T, and the final weight, respectively. Figure 2 shows the x–T curves of oil shale at different heating rates (10℃/min, 15℃/min, 20℃/min, 25℃/min, and 30℃/min). Since the chemical bonds in the oil shale break more easily at higher temperature, increase in the conversion rate with temperature is expected. TG curves shifted to higher temperature region at higher heating rates.

The fraction with the size of less than 150 meshes was taken as the experimental sample. There were no concentration gradient and temperature gradient in the interior of the particle during the reaction. The pyrolysis process was considered to be the intrinsic reaction. The kinetic equation for oil shale pyrolysis is given as follows $\frac{d x}{d t} = kf (x) = A \exp (- \frac{E}{RT}) f (x) = A \exp (- \frac{E}{RT}) (1 - x)^{n}$ (2) where t is the pyrolysis time, x is the conversion rate, f(x) is a function, the type of which depends on the reaction mechanism. The temperature-dependent rate constant, k is usually described by Arrhenius equation. A is the frequency factor, E is the activation energy, R is the universal gas constant, and T is the absolute temperature.

This equation expresses the fraction of the material consumed in the given time. In this work, the key is to determine “three dynamic factors”, which are the activation energy E, frequency factor A, and f(x).

Calculation of E

The activation energy was obtained from the nonisothermal and isothermal TG.

Calculation of E can be obtained by Friedman method, FWO method, KAS method (Braun, 1975; Feng et al., 1997; Zhao et al., 1991) and CR method (Brown et al., 2000; Fu, 1995; Liu et al., 2011; Qian and Yin, 2008; Xiong et al., 2012).

Coats–Redferm method

The graphical method developed by Coats and Redfern was used to evaluate the kinetic data from the thermogravimetric curves. The following equations were used for analysis.

When n = 1:

Using log function on both sides of equation (2) will give the following equation $\ln [\frac{- \ln (1 - x)}{T^{2}}] = \ln \frac{AR}{β E} (1 - \frac{2 RT}{E}) - \frac{E}{R} \frac{1}{T}$ (3)

When n ≠ 1:

Using log function on both sides of equation (2) will give equation (4) $\ln [\frac{(1 - x)^{1 - n} - 1}{(n - 1) T^{2}}] = \ln \frac{AR}{β E} (1 - \frac{2 RT}{E}) - \frac{E}{R} \frac{1}{T}$ (4)

For nonisothermal measurements with a linear heating rate of β = dT/dt, the term 2RT/E < <1 can be omitted. With this simplification, the equation (1–2RT/E) is equal to 1. A plot of the left of the equation versus (1/T) at different orders of reaction (n = 0.5, 1, 1.5, 2) will give a straight line for the correct value of n. The value n = 1 offers the best regression line. So the decomposition of oil shale is the first-order reaction. The plots for the integral method at various heating rates can be obtained. Activation energy can be determined from the slope and intercept of line, respectively.

Friedman method $\ln \frac{d x}{d T} = \ln (\frac{A}{β} f (x)) - \frac{E}{R} \frac{1}{T}$ (5)

Flynn–Wall–Ozawa method $\log β = \log (\frac{AE}{G (x)}) - 2.315 - 0.4567 \frac{E}{R} \frac{1}{T}$ (6)

Kissinger–Akahira–Sunose method $\ln \frac{β}{T^{2}} = \ln (\frac{AR}{EG (x)}) - \frac{E}{R} \frac{1}{T}$ (7)

Calculation of f(x)

$\ln \frac{d x / d T}{f (x)} = \ln (\frac{A}{β}) - \frac{E}{R} \frac{1}{T}$ (8)

In order to calculate f(x), the Sestak reaction equation is introduced, where n, m, and p are partial reaction order terms $f (x) = x^{m} (1 - x)^{n} [- \ln (1 - x)]^{p}$ (9) x^m, (1−x) ⁿ , [−ln(1−x)] ^p represents the proliferation, interface, and decomposition mechanisms, respectively.

Calculation of A

The values of A can be calculated by plotting the experimental values of the left of equation against 1/T.

Calculation of kinetics parameter

Calculation of the activation energy E

Coats–Redferm method

The calculated activation energy for different heating rates is shown in Table 3. From the data in the table, it was found that the value of activation energy decreases little with the increase in the heating rate. The value of active energy is distributed between 199 and 212 kJ/mol. The average active energy is 205.28 kJ/mol.

Friedman method, Flynn–Wall–Ozawa method, and Kissinger–Akahira–Sunose method

Table 3.

Calculation of E by Coats–Redferm method.

Heating rate (℃/min)	10	15	20	25	30	Value
E (kJ/mol)	199.69	203.40	205.98	204.83	212.51	205.28

In the present study, three methods were applied for the determination of the activation energy: Friedman method, Flynn–Wall–Ozawa method, and Kissinger-Akahira-Sunose method. From the data in Table 4, it was found that the value of activation energy ranged from 42.9 to 114.7 kJ/mol. Comparison of these methods and Coats–Redfern method of kinetic analysis revealed very similar results for the Mudanjiang oil shale. Figure 4 shows that the Coats–Redfern equation gave lower activation energy compared to these methods, which indicates that the pyrolysis process of oil shale is very complicated.

Table 4.

Calculation of E.

Method	Friedman	FWO	KAS	Value
E (kJ/mol)	278.01	203.40	205.98	264

The pyrolysis process of oil shale is very complicated, so the reaction order model is not suitable to descript the pyrolysis process of oil shale. It is assumed that oil shale is the mixture of complex molecules of many functional groups and chained bonds. So the active energy is constantly changing. This method avoids the choice of f(x), which minimizes the calculation error.

Calculation of f(x)

The fitting reaction equation is given as follows $f (x) = x^{- 7} (1 - x)^{5} [- \ln (1 - x)]^{7}$ (10)

The value of P is big, which indicates that the nuclear reaction is the main control mechanism. Because oil shale kerogen is a complicated polymer with a kind of three-dimensional structure, the pyrolysis process of oil shale becomes very complex, including many parallel reactions and the reactions in series. These reactions involve the rupture of different chemical bonds with different energies.

Calculation of A

The calculation of A value of different heating rates are given. The average value is 4.11 as given in Table 5.

Table 5.

Calculation of A.

Heating rate (℃/min)	10	15	20	25	30	Value
A/10²¹	3.82	3.91	4.05	4.22	4.57	4.11

The kinetic model of thermal decomposition is given as follows $\frac{d x}{d T} = 4.11 \times 10^{21} \exp (- \frac{264}{8.314 T}) x^{- 7} (1 - x)^{5} [- \ln (1 - x)]^{7}$ (11)

The conversion of hydrocarbon at different temperatures can be calculated by the kinetic model, which provides reference data for the design of retort. The required time for the same conversion at the constant temperature can be calculated by the reaction dynamics mathematical model.

Conclusions

The characteristics of Mudanjiang oil shale is “three-high and one-low”. There are mainly three kinds of chemical reactions in the pyrolysis process of oil shale. Kinetic mathematical model of thermal decomposition are proposed based on the data of Rock-Eval analysis. The required time for the same conversion and the conversion of hydrocarbon at different temperatures can be calculated by the mathematical model.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This study was funded by The National Basic Research Program (973) of China (No.2013CB228000) and Chinese State Municipal Science and Technology Project (No.2017ZX05035).

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Pyrolysis mechanism and kinetic model of Mudanjiang oil shale in the Songliao Basin,China

Abstract

Keywords

Industrial property of Mudanjiang oil shale

Pyrolysis mechanism of oil shale

Reaction of aliphatics

Reaction of aromatics

Reaction of heteroatomic functional group

Kinetics of thermal decomposition

Discussion

Kinetics of the thermal decomposition

Calculation of E

Calculation of f(x)

Calculation of A

Calculation of kinetics parameter

Calculation of the activation energy E

Calculation of f(x)

Calculation of A

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References